Murine Chronic Respiratory Disease (MCRD) provides an unusual opportunity for the study of the pathogenesis of chronic pulmonary inflammation. The recognized morphologic similarities and similar natural histories of chronic bronchitis, bronchiectasis, and emphysema in man and MCRD, due to Mycoplasma pulmonis, in rats and mice provides the basis for use of this model. The genetic differences in susceptibility between different inbred strains further strengthens its usefulness. While data obtained using this model may not be directly applicable to man, they should provide insights into basic and essentially unexplored pathogenetic mechanisms of chronic lung disease. Immunofluorescence, immunoelectronmicroscopy, radioautography, and microcytoxicity will be used in this model to: 1) Completely characterize the reactive cell types in normal and diseased lungs; 2) define the functional capability of these cells for protection and their role in lesion production; and 3) investigate a possible mechanism whereby M. pulmonis evades the host response, thereby inducing chronic active inflammation. Furthermore, these studies will determine the relative contributions of resident and systemic cells in this inflammation.